I usually play through the original vanilla game before deciding to buy DLC, unless it's a game I KNOW for sure I'm going to play a lot of. As for best DLC, nothing really comes to mind. Most of the DLC stuff I'm come across is pretty passable.

Worst DLC... map packs. Problem is if a server is using new maps and you don't have them downloaded, too bad, you can't play on that server. So unless you keep up with the map pack releases, you will find yourself quickly excluded from a lot of servers, which isn't fun.